MEDIUM: ssl/ocsp: counters for OCSP stapling

MEDIUM: ssl/ocsp: counters for OCSP stapling

Add 2 counters in the SSL stats module for OCSP stapling.

- ssl_ocsp_staple is the number of OCSP response successfully stapled
  with the handshake
- ssl_failed_ocsp_stapled is the number of OCSP response that we
  couldn't staple, it could be because of an error or because the
  response is expired.

These counters are incremented in the OCSP stapling callback, so if no
OCSP was configured they won't never increase. Also they are only
working in frontends.

This was discussed in github issue #2822.